The present invention relates to optical filters, and more particularly, to tunable Fabry-Perot optical resonators, and filters and lasers constructed therefrom.
Tunable optical resonators are utilized in optical communication systems and in the construction of lasers. Optical filters and lasers based on Fabry-Perot resonators can be constructed using microelectromechanical (MEM) machining techniques, and hence, in principle, can provide an economically attractive tunable filter or tunable laser. In such devices, a Fabry-Perot resonator cavity is formed between two mirrors. One of these mirrors is flat and located on a semiconductor substrate. The other mirror may be curved and is suspended on a number of micro-mechanical cantilevers. Application of a tuning voltage between the cantilevers and the substrate causes the suspended mirror to move towards the fixed mirror on the substrate, thereby reducing the spacing between the two mirrors of the Fabry-Perot resonator. Since the filter""s bandpass frequency is determined by the mirror spacing, a reduction in spacing between the two mirrors causes the resonant optical frequency of the cavity to increase. The shift in the resonant frequency enables the device to be used directly as a tunable bandpass filter. If an optically-pumped or electrically-pumped optical gain medium (active region) is placed in the cavity, the device becomes a tunable laser, with the lasing wavelength controlled by the resonant frequency of the Fabry-Perot cavity.
In many prior art MEM Fabry-Perot resonators the tuning voltage needed to provide a specific resonance frequency depends on the power in the light that is resonating within the cavity. This power dependent tuning makes the use of such resonators difficult in applications in which the power level in the resonator changes with time.
Broadly, it is the object of the present invention to provide an improved MEM Fabry-Perot resonator.
It is a further object of the present invention to provide a MEM Fabry-Perot resonator having a tuning voltage that is independent of the optical power level in the resonator.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a tunable optical resonator constructed from a fixed mirror and a moveable mirror. The fixed mirror is attached to a substrate having a first electrically conducting surface. A support member having the moveable mirror supported thereon and a second electrically conducting surface is suspended above the substrate such that the moveable mirror is separated from the fixed mirror. A light output port transmits a light signal of a wavelength determined by the distance between the fixed and moveable mirrors. An optical circuit measures the power level of the light signal and generates an electrical signal that depends on the measured power level. A frequency adjustment circuit, responsive to the electrical signal and a tuning voltage, applies an electrical potential between the first and second electrically conducting surfaces. The electrical potential causes the distance to remain at a value determined by the tuning voltage independent of the power level for power levels less than a predetermined power level. The resonator can be used to construct a laser by incorporating an active layer for amplifying light trapped in the cavity. The resonator can also be used as a tunable optical filter by including an input port for receiving a light signal that is to be filtered. In one embodiment of the invention, a feedback circuit consisting of an optical coupler, photodiode, amplifier, and signal adder is used to generate the potential between the first and second electrically conducting surfaces.